1. Field of the Invention
The present invention relates to a substrate with a piezoelectric thin film, particularly to a Pb-free substrate with a piezoelectric thin film.
2. Related Art
A piezoelectric element having a piezoelectric material is used for an actuator or a sensor etc. A Pb(Zr1-xTix)O3 based perovskite-type ferroelectric substance (piezoelectric lead zirconium titanate: PZT) has been widely used as a piezoelectric material for forming such a piezoelectric element. However, since the PZT contains lead (Pb), it is desired to use a piezoelectric material containing no lead, i.e. a Pb-free piezoelectric material for a piezoelectric element from a consideration of environmental point of view.
As a conventional Pb-free piezoelectric thin film element, for instance, there is a piezoelectric thin film element using a dielectric film comprising an alkali niobium oxide based perovskite compound. This piezoelectric thin film element comprises a substrate comprising MgO etc., a lower electrode formed on the substrate, a buffer layer comprising BaTiO3 etc. and formed on the lower electrode, a piezoelectric thin film comprising an alkali niobium oxide based perovskite compound expressed by a general formula of (NaxKyLiz)NbO3 (0<x<1, 0<y<1, and x+y+z=1) and formed on the buffer layer, and an upper electrode formed on the piezoelectric thin film. For instance, Japanese Patent Laid-Open No. 2007-19302 (JP-A-2007-19302) discloses this type of Pb-free piezoelectric thin film element.
However, in the piezoelectric thin film element according to JP-A-2007-19302, a warping might be generated in the piezoelectric thin film element when forming the piezoelectric thin film comprising a potassium sodium niobate based material on the substrate of MgO etc. If a piezoelectric operation is continuously conducted for a long period in such a warped piezoelectric thin film element, a piezoelectric constant d31 of the piezoelectric thin film element will be decreased greatly.